1. Field of the Invention
The present invention is directed to a prosthetic cup assembly that is disclosed in the context of a hip prosthesis.
2. Background Information
It is known to provide an acetabular cup assembly that includes a metal shell component for attachment to an acetabulum to replace the natural socket and a plastic bearing component that is inserted into the shell to provide a bearing surface for receiving a femur ball prosthesis element. See for example, U.S. Pat. No. 5,049,158, to John A. Englehardt et al., the disclosure of which is expressly incorporated herein by reference. In addition, traditional bearing components include a built-up lip around a portion of the bearing surface. See for example, U.S. Pat. Nos. 5,288,864 and 5,413,603 to Noiles et al., the disclosures of which are also expressly incorporated herein by reference.
A problem that can occur with such acetabular cup assemblies is motion between the outer metal shell and the plastic bearing component or insect. Motion between the outer metal shell and the plastic bearing insert causes wear and thus the possibility of wear debris particles. Wear debris particles have been associated with particle-induced osteolysis. In view of this, it is desirable to reduce or eliminate motion between the metal shell and the plastic insert of acetabular cup assemblies.
Previous acetabular cup assembly designs have focused on macroscopic motion between the metal shell and the plastic insert. Other designs have sought to decrease the amount of particles generated by such motion by decreasing the surface finish at the surface of interaction between the metal shell and the plastic insert (see U.S. Pat. No. 5,310,408 issued to Schryver et al.). Still further, acetabular cup assembly designs have focused on using a third member as a way to maintain macrostability of the assembly parts while maintaining dome loading. Dome loading designs essentially ensure contact in the dome region by leaving clearance under the lip of the liner. These dome loading designs however, cause the insert to seat in the direction of the applied load.
Referring to FIGS. 20 and 21, there is shown a prior acetabular cup assembly, generally designated 500. The prior acetabular cup assembly 500 represents a typical dome loading ring lock design and includes a shell 502 and a bearing insert or liner (not seen in the figures) that is disposed interior of the shell 502. The portion of the acetabular cup assembly 500 that is shaded, depicts or represents congruency between the liner and the shell 502 of the acetabular cup assembly 500 when a load is applied to the inside of the liner at 20° relative to an axis defined from the center opening 504. The shaded portion of the shell 502 may thus be considered a load pattern and is generally designated 506. Thus, the remaining portion of the shell 502 that is not shaded represents non-congruency between the liner and the shell 502.
In particular, prior dome loading ring lock designs primarily load on a spherical surface. This causes the liner (typically polyethylene) to seat against the inner surface of the shell 502 in the direction of the applied load. As the applied load shifts from one direction to another direction, the loading pattern 506 shifts about the shell. This shifting motion applied to the liner that is then in turn applied to the shell causes wear on the liner.
It is thus desirable to reduce and/or obviate the above-described condition in a prosthetic component assembly. It is further desirable to provide a prosthetic component assembly that has increased congruency between a liner and shell of the prosthetic component assembly. It is also desirable to provide a prosthetic component assembly that maintains an increased congruency between a liner and a shell thereof regardless of loading induced on the liner. It is still further desirable to provide a prosthetic component assembly that provides controllable motion between a liner and a shell of the prosthetic component assembly. It is yet further desirable to provide a prosthetic component assembly that provides uniform controllable motion between a liner and a shell thereof.